This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-033195, filed Feb. 10, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a diode defect detecting device of a DC load supply apparatus used in, e.g., an inverter type resistance welding machine.
FIG. 1 shows an example of a controller of a conventional inverter type resistance welding machine.
An inverter circuit in which IGBT switching elements 2, 3, 4, and 5 are bridge-connected is connected in parallel with a DC power supply 1, thereby converting a direct current into an alternate current. This alternate current is supplied to the primary side of a transformer 7.
A load resistance 11 is connected to the secondary side of the transformer 7. An alternate current on this secondary side is rectified into a direct current by a rectifying circuit in which a plurality of diodes 8 and 9 are connected in a center tap manner. This direct current is supplied to the load resistance 11.
A current sensor 6 senses the primary current of the transformer 7. A current detector 15 detects this current after converting it into a direct current. An overcurrent detector 16 detects an inverter output overcurrent from the output from the current sensor 6.
The diodes 8 and 9 convert the secondary alternate current of the transformer 7 into a direct current and supplies the current to the load resistance 11 via a wiring inductance 10. This wiring inductance 10 is not particularly formed; it represents the inductance present in wiring.
An output current reference value from a current reference unit (current reference circuit) 12 is input to an amplifier 14 when a start switch 13 is turned on. This amplifier 14 amplifies any deviation of the output from the current detector 15 from the output from the current reference unit 12. This amplified deviation and a triangular-wave frequency output from a triangular-wave generator 17 are input to a PWM circuit 18. On the basis of these outputs from the amplifier 14 and the triangular-wave generator 17, the PWM circuit 18 outputs a PWM signal. This PWM signal is applied to the gates of the switching elements 2 to 5 via a polarity changing circuit 19 in order to switch the polarity of the inverter output.
As described above, when the diodes 8 and 9 are normal the current control is performed such that the peak value of the primary current of the transformer 7 corresponds to the current reference.
FIG. 2 is a timing chart for explaining an operation when only the diode 8 is shorted in FIG. 1. At time t1, the start switch 13 is turned on to apply an inverter output voltage V1 having pulse widths t1 and t2 to the primary side of the transformer 7.
Consequently, on the secondary side of the transformer 7 a transformer primary current I1 rises in a circuit of diode 8 (shorted)xe2x86x92wiring inductance 10xe2x86x92load resistance 11. When the inverter output voltage V1 is shut off at time t2, the transformer primary current I1 becomes zero.
Next, the inverter output voltage V1 is inverted at time t3. Since the transformer secondary side is shorted by a circuit of diode 9xe2x86x92diode 8 (shorted), the transformer primary current I1 rapidly rises. At time t4 at which this transformer primary current I1 reaches the detection level of the overcurrent detector 16, the overcurrent detector 16 detects an overcurrent, and the inverter stops operating.
If the pulse width of the inverter output voltage is smaller than the interval between t3 and t4, the inverter sometimes keeps operating because the overcurrent level is not reached.
The conditions are affected by the magnitude of the current reference or the response (generally, PI control is used) speed of the amplifier 14.
As described above, when the diodes 8 and 9 are short-circuited in the controller of the conventional inverter type resistance welding machine, depending on the conditions the overcurrent detector 16 can detect an overcurrent or the overcurrent detection level is not reached. This makes the detection unstable. At the same time, the inverter operates at a current slightly lower than the overcurrent detection level. This applies stress on the switching elements 2 to 5 constructing the inverter and may degrade the reliability of the inverter. Furthermore, where the diodes are shorted, a normal current can not flow into a welding object, thus a problem arises that defective items flows to following process.
It is an object of the present invention to provide a diode defect detecting device capable of reliably detecting defects of diodes without applying the stress to switching elements of an inverter, thereby improving reliability of the device and detecting a defect of the diode at an earliest stage.
To achieve this object, according to the first aspect of the present invention, there is provided a diode defect detecting device used in a controller in which an inverter converts a direct current into an alternate current and supplies the alternate current to a primary side of a transformer, and a rectifying circuit having a plurality of diodes connected to a secondary side of the transformer rectifies an alternate current induced on the secondary side into a direct current and supplies the direct current to a load, comprising:
a current detector for detecting a primary current of the transformer;
a comparator for comparing the detection current detected by the current detector with a current reference and outputting a reset signal if the detection current is larger than the current reference;
an oscillator for generating a clock signal;
a flip-flop circuit for receiving a set signal on the basis of a front and an end edge of the clock signal generated by the oscillator and receiving the output reset signal from the comparator;
a polarity changing circuit for outputting a polarity changing signal for changing polarities of the plurality of diodes on the basis of the output clock signal from the flip-flop circuit; and
a determination circuit for calculating, in order to detect malfunctions of the plurality of diodes, an impedance change on the secondary side viewed from the primary side of the transformer on the basis of a pulse width of each polarity of the output clock signal from the flip-flop circuit.
According to the second aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit calculates the impedance change by using a modulation factor of the pulse width of each polarity and a peak value of the primary current detected by the current detector, thereby detecting a diode malfunction.
According to the third aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit calculates the impedance change by using a modulation factor of the pulse width of each polarity and the current reference, thereby detecting a diode malfunction.
According to the fourth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit detects a diode malfunction when the modulation factor of the pulse width of each polarity becomes equal to or smaller than a predetermined value.
According to the fifth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit obtains a pulse width of each polarity during a few cycles after the inverter is started.
According to the sixth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the determination circuit detects a diode malfunction if a value obtained by dividing a pulse width of one polarity by a pulse width of the other polarity is larger than a predetermined value.
According to the seventh aspect of the present invention, there is provided a diode defect detecting device of the first aspect, further comprising:
an alarm generating circuit for generating alarm when the determination circuit detects a diode malfunction.
According to the eighth aspect of the present invention, there is provided a diode defect detecting device of the first aspect, wherein the pulse width of each polarity is obtained by counting the pulse width of each polarity of the output clock signal from the flip-flop circuit by using a counter.
According to the ninth aspect of the present invention, there is provided a diode defect detecting device used in a controller in which an inverter converts a direct current into an alternate current and supplies the alternate current to a primary side of a transformer, and a rectifying circuit having a plurality of diodes connected to a secondary side of the transformer rectifies an alternate current induced on the secondary side into a direct current and supplies the direct current to a load, comprising:
a current detector for detecting a primary current of the transformer;
a comparator for comparing the detection current detected by the current detector with a current reference and outputting a reset signal if the detection current is larger than the current reference;
an oscillator for generating a clock signal;
a flip-flop circuit for receiving a set signal on the basis of a front and an end edge of the clock signal generated by the oscillator and receiving the output reset signal from the comparator;
a polarity changing circuit for outputting a polarity changing signal for changing polarities of the plurality of diodes on the basis of the output clock signal from the flip-flop circuit;
a current change rate detector for detecting a change rate of the primary current of the transformer;
a first latch circuit for latching the current change rate detected by the current change rate detector at an input timing of an end edge of a positive side clock signal of the clock signal of each polarity output from the flip-flop circuit;
a second latch circuit for latching the current change rate detected by the current change rate detector at an input timing of an end edge of a negative side clock signal of the clock signal of each polarity output from the flip-flop circuit; and
a circuit for detecting malfunctions of the plurality of diodes if a difference between the current change rates latched by the first and second latch circuits is equal to or larger than a predetermined value.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.